FIG. 1 shows a network structure of the E-UMTS, a mobile communication system, applicable to the related art and the present invention. The E-UMTS system has been evolved from the UMTS system, for which the 3GPP is proceeding with the preparation of the basic specifications. The E-UMTS system may be classified as the LTE (Long Term Evolution) system.
The E-UMTS network may be divided into an evolved-UMTS terrestrial radio access network (E-UTRAN) and a core network (CN). The E-UTRAN includes a terminal (referred to as ‘UE (User Equipment), hereinafter), a base station (referred to as an eNode B, hereinafter), a serving gateway (S-GW) located at a termination of a network and connected to an external network, and a mobility management entity (MME) superintending mobility of the UE. One or more cells may exist for a single eNode B.
FIGS. 2 and 3 illustrate a radio interface protocol architecture based on a 3GPP radio access network specification between the UE and the base station. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting control signals (signaling). The protocol layers can be divided into the first layer (L1), the second layer (L2), and the third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in communication systems.
The radio protocol control plane in FIG. 2 and each layer of the radio protocol user plane in FIG. 3 will now be described.
The physical layer, namely, the first layer (L1), provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel, and data is transferred between the MAC layer and the physical layer via the transport channel. Meanwhile, between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transferred via a physical channel.
The MAC layer of the second layer provides a service to a radio link control (RLC) layer, its upper layer, via a logical channel. An RLC layer of the second layer may support reliable data transmissions. A PDCP layer of the second layer performs a header compression function to reduce the size of a header of an IP packet including sizable unnecessary control information, to thereby effectively transmit an IP packet such as IPv4 or IPv6 in a radio interface with a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer is defined only in the control plane and handles the controlling of logical channels, transport channels and physical channels in relation to configuration, reconfiguration and release of radio bearers (RBs). The radio bearer refers to a service provided by the second layer (L2) for data transmission between the UE and the UTRAN.
A random access channel (RACH) will now be described. The RACH is used to transmit data with a relatively short length to uplink, and in particular, the RACH is used when a UE, which has not been allocated dedicated radio resources, has a signaling message or user data to be transmitted to uplink. Or, the RACH may be also used for a base station to instruct a UE to perform a RACH procedure.
A random access channel (RACH) procedure provided by the LTE system will now be described. The RACH procedure provided by the LTE system is divided into a contention-based RACH procedure and a non-contention-based RACH procedure. The contention-based RACH procedure and the non-contention-based RACH procedure are determined based on whether or not a random access preamble used in a RACH procedure has been directly selected by a UE or by a base station.
In the non-contention-based RACH procedure, the UE uses a random access preamble the base station has directly allocated to the UE. Thus, when the base station allocates the particular random access preamble only to the UE, the random access preamble is used by only the UE while other UEs do not use it. Thus, a one-to-one relationship is established between the random access preamble and the UE using the random access preamble, so there is no collision. This is effective because the base station can recognize the UE that has transmitted the random access preamble upon receiving the random access preamble.
Meanwhile, in the contention-based RACH procedure, the base station selectively transmits one of random access preambles, so there is a possibility that a plurality of UEs may use the same random access preamble. Thus, when the base station receives a certain particular random access preamble, it cannot recognize which UE has transmitted the random access preamble.
In general, the UE may perform the RACH procedure in the following cases: 1) hen the UE performs initial accessing because it is not RRC-connected with the base station, 2) when the UE is first connected to a target cell during a handover process, 3) when the RACH procedure is requested by an instruction of the base station, 4) when data to uplink is generated in a state that time synchronization of uplink is not matched or in a state that designated radio resources used for requesting radio resources have not been allocated, and 5) when a recovery process is performed in case of a radio link failure or a handover failure.
FIG. 4 shows operations of the UE and the base station in the contention-based RACH procedure.
First, in the contention-based random access, the UE randomly selects one random access preamble from a set of random access preambles instructed by system information or a handover command, selects PRACH resource that can transmit the random access preamble, and transmits the same (first step). The preamble at this time is called an RACH MSG 1.
After transmitting the random access preamble, the UE attempts receiving of a response to its random access preamble within a random access response reception window instructed by the system information or the handover command (second step). In more detail, random access response information is transmitted in the form of MAC PDU, and the MAC PDU may be transferred via a physical downlink shared channel (PDSCH). In addition, in order for the UE to properly receive information transmitted via the PDSCH, a physical downlink control channel (PDCCH) is also transferred. Namely, the PDCCH may include information about the UE which is to receive the PDSCH, frequency and time information of radio resources of the PDSCH, a transmission format of the PDSCH, and the like. Here, when the UE successfully receives the PDCCH which has been transmitted thereto, it properly receives the random access response transmitted via the PDSCH according to the information of the PDSCH. The random access response includes a random access preamble identifier (ID), a UL grant (uplink radio resources), a temporary C-RNTI (temporary cell identifier), and a time alignment command (time synchronization correction value). The reason why the random access preamble ID is required is because one random access response may include random access response information for one or more UEs, so the random access preamble ID informs about for which UE the UL grant, the temporary C-RNTI and the time alignment command information are valid. The random access preamble ID is identical to the random access preamble that has been selected by the UE itself.
Here, when the UE receives the random access response valid for the UE itself, the UE processes information included in the random access response. Namely, the UE applies the time alignment command and stores the temporary C-RNTI. In addition, the UE transmits data stored in its buffer or newly generated data to the base station (third step). In this case, data (referred to as ‘MSG 3’, hereinafter) included in the UL grant should necessarily include an identifier of the UE. The reason is because, in the contention-based RACH procedure, the base station can hardly determine which UEs perform the RACH procedure, and it should identify UEs to prevent occurrence of collision. Here, there are two methods for including the ID of the UE. The first method is that when the UE already has a valid cell ID which has been allocated in a corresponding cell before the RACH procedure, the UE transmits its cell ID via the UL grant. If, however, the UE has not been allocated a valid cell ID before the RACH procedure, the UE includes its unique ID (e.g., an S-TMSI or a random ID) and transmits the same. In general, the unique ID is longer than the cell ID. In the third step, when the UE transmits data via the UL grant, the UE starts a contention resolution timer.
After the UE transmits the data including its ID via the UL grant included in the random access response, the UE waits for an instruction of the base station to resolve contention. Namely, the UE attempts receiving of the PDCCH to receive a particular message (a fourth step). Here, there are two methods for receiving the PDCCH. As mentioned above, if the identifier of the UE transmitted via the UL grant is a cell ID of the UE, the UE attempts receiving of the PDCCH by using its cell ID, and if the identifier is its unique ID, the UE attempts receiving of the PDCCH by using the temporary C-RNTI included in the random access response. Thereafter, in the former case, if the UE receives the PDCCH (referred to as ‘MSG 4’, hereinafter) via its cell ID before the contention resolution timer expires, the UE determines that the RACH procedure has been normally performed, and terminates the RACH procedure. In the latter case, if the PDCCH is received via the temporary cell ID before the contention resolution timer expires, the UE checks data (referred to as ‘MSG 4’, hereinafter) transferred by the PDSCH indicated by the PDCCH. If content of the data includes its unique ID, the UE determines that the RACH procedure has been normally performed and terminates the RACH procedure. Here, the message or the MAC PDU received in the fourth step is usually called RACH MSG 4.
A method for receiving downlink data by the UE in the LTE system will now be described. FIG. 5 illustrates allocation or radio resources according to the related art.
In the downlink direction, physical channels are divided into the physical downlink control channel (PDCCH) and the physical downlink shared channel (PDSCH). The PDCCH is not directly related to transmission of user data and transmits control information required for operating a physical channel. Briefly, the PDCCH may be used to control other physical channels. In particular, the PDCCH is used to transmit information required for receiving the PDSCH. Information such as for which UE data is designated to be transmitted by using a particular frequency band at a particular point, which size of data is transmitted, and the like, is transmitted via the PDCCH. Thus, each UE receives the PDCCH at a particular TTI and checks whether or not data to be received by the UE is transmitted via the PDCCH. If it is informed that data to be received by the UE is transmitted, the UE additionally receives the PDSCH by using information such as frequency indicated by the PDCCH. Information about to which UE (one or a plurality of UEs) data of the PDSCH is transmitted or how the UEs receive the PDSCH data and decode it, and the like, may be included in a physical PDCCH and transmitted.
For example, it is assumed that, in a particular sub-frame, radio resource information (e.g., a frequency position) called ‘A’ and transmission format information (e.g., transport block size, modulation and coding information, etc.) called ‘B’ are CRC-masked to an RNTI (Radio Network Temporary Identity) called ‘C’, and transmitted via the PDCCH. One or two or more UEs located in a corresponding cell monitor the PDCCH by using their RNTI information, and on the above assumption, when the UE having the RNTI called ‘C’ decodes the PDCCH, a CRC error does not occur. Thus, the UE decodes the PDSCH to receive the data by using the transmission format information called ‘B’ and the radio resource information called ‘A’. Meanwhile, on the above assumption, if the UE does not have the RNTI called ‘C’, when the PDCCH is decoded, a CRC error occurs. Thus, the UE does not receive the PDSCH.
In the above procedure, the RNTI (Radio Network Temporary Identifier) is transmitted to inform to which UEs radio resources have been allocated. The RNTI includes a dedicated RNTI and a common RNTI. The dedicated RNTI is allocated to a single UE and used to transmit/receive data corresponding to the UE. The dedicated RNTI is allocated only to a UE whose information has been registered in the base station. Meanwhile, the common RNTI is used when UEs, which have not been allocated the dedicated RNTI because their information was not registered to the base station, transmit or receive data to or from the base station, or the common RNTI is used to transmit information commonly applied for a plurality of UEs.
As mentioned above, the base station and the UE are two main entities constituting the E-UTRAN. Radio resources in a single cell include uplink radio resources and downlink resources. The base station handles allocating and controlling of uplink and downlink radio resources and downlink radio resources of the cell. Namely, the base station determines which UE uses which radio resources at which moment. For example, the base station may determine to allocate frequency from 100 Mhz to 101 Mhz to a user 1 for downlink data transmission in 3.2 seconds. After such determination, the base station informs the UE accordingly so that the UE can receive downlink data. Also, the base station may determine when and which UE is allowed to transmit uplink data by using which and how many radio resources, and then informs a corresponding UE accordingly, so that the UE can transmit data by using the radio resources for the corresponding time. In the related art, a single terminal keeps using a single radio resource during a call connection, which is irrational for the recent services which are mostly based on IP packets. That is, in most packet services, packets are not continually generated during a call connection but there are intervals in the call during which none is transmitted. Thus, continuously allocating radio resources to the single terminal is ineffective. To solve this problem, the E-UTRAN system employs a method in which radio resources are allocated to the UE in the above-described manner only when the UE requires it or only when there is service data.
A semi-persistent scheduling (or a semi-persistent radio resource allocation method) or semi-permanent scheduling (or a semi-permanent radio resource allocation method) will now be described. In general, the UE transmits data to the base station through the process including: 1) the UE requests radio resources required for transmitting generated data from the base station, 2) the base station allocates radio resources through a control signal according to the UE request for radio resources, and 3) the UE transmits the data to the base station through the allocated radio resources. However, in the VoIP service, in general, small packets of uniform size are frequently and regularly transmitted. So, the effective radio resource allocation scheme can be applied in consideration of such characteristics. Namely, the semi-permanent scheduling is also one of radio resource allocation schemes optimized for a VoIP service. In this method, transmission of information regarding allocation of radio resources is omitted. In more detail, when VoIP starts, A packet size and period of RTP are previously determined and radio resources are permanently allocated. Accordingly, the UE may immediately perform the process of transmitting data without the first and second steps, namely, without the radio resource requesting step and the radio resource allocation step, as mentioned above, according to such setting of resource resources. That is, in the semi-persistent scheduling, there is no need to transmit radio resource allocation information via a PDCCH. Without receiving the PDCCH each time, the UE can periodically receive particular radio resources or transmit data by using particular radio resources according to pre-set information. Meanwhile, the dynamic scheduling is a method for informing about radio resources to be received or to be transmitted by the UE each time.
The base station may selectively set a dedicated scheduling request channel (D-SR Channel) for the UE. The D-SR channel may transmit 1-bit information at uniform time intervals.
In more detail, in the LTE system, in order to effectively use radio resources, the base station should know which and how many data each user wants to transmit. In case of downlink data, the downlink data is transferred from an access gateway to the base station. Thus, the base station knows how many data should be transferred to each user through downlink. Meanwhile, in case of uplink data, if the UE does not directly provide the base station with information about data the UE wants to transmit to uplink, the base station cannot know how many uplink radio resources are required by each UE. Thus, in order for the base station to appropriately allocate uplink radio resources to the UEs, each UE should provide information required for the base station to schedule radio resources to the base station.
To this end, when the UE has data to be transmitted, it provides corresponding information to the base station, and the base station transfers a resource allocation message to the UE based on the received information.
In this process, namely, when the UE informs the base station that it has data to be transmitted, the UE informs the base station about the amount of data accumulated in its buffer. It is called a buffer status report (BSR).
The BSR is generated in the format of a MAC control element, included in a MAC PDU, and transmitted from the UE to the base station. Namely, uplink radio resources are required for the BSR transmission, which means that uplink radio resource allocation request information for BSR transmission should be sent. If there is allocated uplink radio resource when the BSR is generated, the UE would transmit the BSR by using the uplink radio resource. The procedure of sending the BSR by the UE to the base station is called a BSR procedure. The BSR procedure starts 1) when every buffer does not have data and data is newly arrived to a buffer, 2) when data is arrived to a certain empty buffer and a priority level of a logical channel related to the buffer is higher than a logical channel related to the buffer previously having data, and 3) when a cell is changed. In this respect, with the BSR procedure triggered, when uplink radio resources are allocated, if transmission of all the data of the buffer is possible via the radio resources but the radio resources are not sufficient to additionally include the BSR, the UE cancels the triggered BSR procedure.
However, if there is no allocated uplink radio resource when the BSR is generated, the UE performs a scheduling request (SR) procedure (i.e., resource allocation request procedure).
The SR procedure includes two methods: one is using a D-SR (Dedicated Scheduling Request) channel set for a PUCCH, and the other is using a RACH process. Namely, when the SR procedure is triggered and the D-SR channel has been allocated, the UE sends a radio resource allocation request by using the D-SR channel, whereas if the D-SR channel has not been allocated, the UE starts the RACH procedure. In case of using the D-SR channel, the UE transmits a radio request allocation signal on uplink via the D-SR channel.
The SR procedure may be continuously performed until the UE is allocated UL-SCH resources.
In general, with respect to the semi-persistent scheduling, the UE may send any logical channel or any control information by using the radio resources. The radio resources allocated according to the semi-persistent scheduling do not exist at every sub-frame. That is, considering that, in general, voice data is generated at 20 ms intervals, the radio resources are allocated substantially at 20 ms. When a call is established, the base station establishes various types of RBs with the UE. Namely, an RB for exchanging control signaling and an RB for Internet browsing, as well as an RB used for transmission of voice service data, may be set for the UE.
The characteristics of the services set for the RBs and the like are that data for the services are irregularly generated. Data of the voice service is regularly generated at 20 ms intervals, so the time point of generating the data of the voice service and a timing of radio resources allocated according to the semi-persistent scheduling can be adjusted to be consistent. However, a data generation time point of the other RBs are not mostly consistent with the timing of the radio resources allocated according to the semi-persistent scheduling. In this case, the BSR is generated, triggering the SR procedure. The SR procedure takes much time, and if radio resources are allocated according to the semi-persistent scheduling before radio resources are allocated through the SR procedure, the radio resources are wasted.